In vitro studies have shown that non-dividing blood lymphocytes from patients with chronic lymphocytic leukemia are more sensitive than normal lymphocytes to the cytocidal action of many reagents including prednisolone, L-asparaginase, cytarabine, vincristine, and colchicine. As the maximum difference was obtained with colchicine, a pilot study was undertaken to consider the possibility of using this reagent as a test for leukemic lymphocytes. The method used for measuring sensitivity was by a special slide chamber and the counting of viable lymphocytes by phase contrast microscopy. The sensitivity was expressed as the average cytocidal effect produced by 1.0 and 0.1 microgram/ml of colchicine incubated with the cells at 37 degrees C for 20 hours. A high degree of sensitivity of blood lymphocytes was observed for 18 patients with active chronic lymphocytic leukemia, and for 3 patients with presumptive early lymphocytic leukemia; low sensitivity was obtained for one leukemic patient in complete remission, and for 8 normal persons. This possible diagnostic test will be evaluated by studies on treated and untreated leukemic patients, on leukemic patients in partial and complete remission and relapse, on non-leukemic patients with various types of lymphocytosis, and on normal persons. These studies are designed to determine the accuracy, specificity, limitation, and clinical usefulness of the colchicine-sensitivity test for leukemic lymphocytes. Work will also be done to modify and simplify the method of testing, to make it suitable for the clinical laboratory.